As an existing retrieval system, a large-scale retrieval system is known with which plural clients are coupled and which accepts input of retrieval requests often including long retrieval expressions consisting of plural conditional expressions and always executes the retrieval expressions in a multiplex manner. Such a system may not perform another process while performing a process corresponding to the retrieval request for data on which data retrieval is to be performed. Accordingly, when a new retrieval request is input from a client while the retrieval system is performing a process for retrieval, a process corresponding to the new retrieval request may be delayed until the currently performed process ends.
Retrieval expressions which are input to the retrieval system from different clients and which are to be processed simultaneously by the retrieval system often include identical conditional expressions. In the existing retrieval system, when identical retrieval requests are input in this way, a meaningless process of repeating identical retrieval processes is performed and thus a retrieval time extends in proportion to the total number of conditional expressions which include redundant conditional expressions, thereby delaying the process. Conditional expressions may be redundant not only when identical conditional expressions are included in different retrieval expressions but also when identical conditional expressions are included in a single retrieval expression.
Expressions (1-1) to (1-3) represent examples of plural retrieval expressions. Here, A, B, C1, C2, C3, D1, D2, D3, E1, E2, and E3 respectively represent conditional expressions.Retrieval Expression_1=(A and B and C1) OR (A and B and C2) OR (A and B and C3)  (1-1)Retrieval Expression_2=(A and B and D1) OR (A and B and D2) OR (A and B and D3)  (1-2)Retrieval Expression_3=(A and B and E1) OR (A and B and E2) OR (A and B and E3)  (1-3)
Identical conditional expressions A and B are present in Expressions (1-1) to (1-3). Not only that, the conditional expressions A and B appear three times in Expression (1-1).
To address the problem of redundant conditional expressions, a ride-sharing retrieval method of collectively processing plural retrieval expressions has been proposed. The ride-sharing retrieval is one instance of a “multiple instruction stream, single data stream (MISD)” architecture for enhancing the performance on multiplex processing of retrieval expressions. In the ride-sharing retrieval, plural retrieval expressions received for a predetermined time are merged to perform the retrievals. Therefore, a data reference time for the plural merged retrieval processes becomes 1, thus the processing efficiency may be enhanced.
A related technique is disclosed in, for example, Japanese Laid-Open Patent Publication No. 2002-222194.
In an existing retrieval system, when a specific record is extracted from similar data sets, extraction conditions thereof are complicatedly described and thus retrieval conditions may be often highly complicated. In the retrieval system using the ride-sharing retrieval method, since the data reference time for plural retrieval processes may become 1 by merging data reference regarding identical conditional expressions included in the plural retrieval expressions received for a predetermined time to perform the retrievals, thereby enhancing the processing efficiency.
However, in the retrieval system using the ride-sharing retrieval method, the total number of conditional expressions, which are elements of the retrieval expressions, is not changed even when the retrieval expressions are merged. Accordingly, an evaluation time of conditional expressions is not changed before and after the merging. That is, for identical conditional expressions, the data reference time may become efficient but the evaluation time of the conditional expressions does not become efficient in some cases.